Weight painting in Blender 2.8
Weight paint doesn't allow you to paint on vertices that were generated with the multiresolution modifier; it only interpolates the weights for each point based on its neighbors (same as the subsurface modifier). It also won't let you modify the mesh with any modifier besides the armature modifier while you're in the weight painting mode. 

A simple solution would be to simply apply the multiresolution modifier. If you'd like to retain the original (low poly) mesh, you could create a duplicate of that mesh, hide it, and apply the multiresolution modifier on the duplicate mesh.